The unique breadmaking characteristic of wheat flour is closely related to the elasticity and extensibility of the gluten proteins stored in the starchy endosperm, particularly the high molecular weight glutenin subunits (HMW-GS) which are important in determining gluten and dough elasticity. The quality of wheat cultivars depends on the number and composition of the HMW-GS present.
Prolamins are a novel group of storage proteins found in the endosperm of cereal grains (Shewry, 1995). The prolamins of wheat are divided into two groups, gliadins and glutenins. Together, they form gluten, a continuous proteinaceous network, during the mixing of wheat flour with water to make dough. The gluten proteins are the largest protein molecules found in nature (Wrigley, 1996). The elasticity (strength) and extensibility (viscosity) of the dough, critical for breadmaking, are closely related to glutenins and gliadins, respectively. These unique properties of wheat gluten, not found in the storage proteins of other cereals, are likely related to the enormous size of the glutenin polymers which have relative molecular masses ranging into the tens of millions (Wrigley, supra). Low (weak) gluten elasticity is responsible for the poor breadmaking qualities of wheat cultivars which otherwise have desirable agronomic properties. In such instances the mixing of flour from different cultivars is required in order to produce a blend suitable for breadmaking. Extensive biochemical and genetic investigations have shown that the breadmaking quality of wheat flour is determined particularly by the HMW-GS group of proteins. The HMW-GS are subdivided into high M.sub.r x-type and low M.sub.r y-type subunits. Two genes which are inherited as tightly linked pairs, encoding an x-type and a y-type subunit, are present on the 1A, 1B, and 1D chromosomes of hexaploid bread wheat (Payne, 1987). All cultivars of wheat, therefore, contain six HMW-GS genes, but only three, four, or five subunits are present, because some of the genes are silent (the 1 Ay gene is silent in all bread wheat varieties). The number and composition of HMW-GS present in a cultivar are closely related to the quality of its gluten. HMW-GS may represent up to 10% of the total seed protein, as each HMW-GS accounts for about 2% of the total extractable protein (Seilmeier et al., 1991; Halford et al., 1992). However, the close linkage of HMW-GS genes makes it difficult to manipulate them by traditional breeding methods (Flavell et al., 1989). Recent success in the transformation of wheat (Vasil, 1994), therefore, has provided an opportunity to try to improve the gluten quality of wheat by introducing additional copies of HMW-GS genes (Flavellet al., 1989; Shewry et al., 1995). Seeds of transgenic wheat (cv Bob White) containing a hybrid HMW-GS Dy10:Dx5 gene construct have just recently been shown to accumulate the hybrid HMW-GS to levels similar to those of the endogenous HMW-GS genes. Five HMW-GS-Ax2*, Bx7, By9, Dx5, and Dy10--are present in Bob White endosperm (Blechl and Anderson, 1996). The use of the hybrid gene construct was, therefore, necessary to discriminate between the native proteins encoded by the Dx5 and Dy10 genes, and the hybrid HMW-GS formed by the introduced Dy10 and Dx5 genes.
There exists a continuing need for wheat with improved breadmaking quality, and methods for creating such wheat. Therefore, it would be desirable to obtain wheat improved by transformation with heterologous HMW-GS genes which are expressed to yield improved breadmaking quality.